A non-classical type of third-element effect in the oxidation of Cu–xCr–2Al alloys at 1173 K

Oxidation of a Cu–2.2 at.%Al alloy (Cu–2Al) produces external scales containing Cu-oxides plus some alumina with almost no internal oxidation of aluminum. The addition of 3.9 at.%Cr to Cu–2Al has little effect on the oxidation behavior, while addition of 8.1 at.%Cr promotes a fast formation of an alumina layer in contact with the alloy with a simultaneous large reduction in the oxidation rate. This form of third-element effect, not related to the transition from internal to external oxidation of aluminum but to the inhibition of the copper oxidation, is attributed to the complete solubility between alumina and chromia.